The United States Environmental Protection Agency has issued a rule to increases the volume requirements for total renewable fuel to 20.5 billion gallons and for cellulosic biofuel to 3.0 billion gallons by 2015. To meet these mandates, it will be necessary to use cellulosic biomass, an abundant and renewable carbon source, as a feedstock.
Currently, S. cerevisiae is frequently used to ferment biomass sources to produce fuel ethanol. One of the drawbacks in using conventional S. cerevisiae strains is that S. cerevisiae strains typically optimally operate between 25° C. and 30° C. to produce ethanol in industrial fermentation systems. The produced heat from the fermentation system generally requires some means of cooling the system to the optimal temperature range. In the absence of cooling, heat stress on the yeast strain reduces the production yield. Additionally, a yeast strain that optimally ferments at higher temperatures would improve the efficiency of simultaneous saccharification and fermentation, and allow for the continuous ethanol removal by evaporation under reduced pressure. As such, there is a need to develop a heat tolerant yeast strain to take advantage of producing fuel ethanol at higher temperatures.
The yeast strain, Kluyveromyces marxianus, has an advantage over typical Saccharomyces cerevisiae with respect to higher operating temperature ranges in that Kluyveromyces marxianus has been reported to grow at 47° C. and above (Nonklang, S. et al., Appl. Environ. Microbiol. 2008, 74(24), 7514-7521) and produce ethanol at temperatures above 40° C. (Fonseca, G. G., et al., Appl. Microbiol. Biotechnol. 2008, 79, 339-354).
Additionally, K. marxianus offers other benefits compared to typical S. cerevisiae strains. Other benefits include the ability to grow on a wide variety of substrates not utilized by S. cerevisiae such as xylose, xylitol, cellobiose, lactose, arabinose, and glycerol (Nonklang, S.; Abdel-Banat, B. M. A.; Cha-aim, K.; Moonjai, N.; Hoshida, H.; Limtong, S.; Yamada, M.; Akada, R. High-temperature ethanol fermentation and transformation with linear DNA in the thermotolerant yeast Kluyveromyces marxianus DMKU3-1042 Appl. Environ. Microbiol. 2008, 74(24), 7514-7521 and Rodrussamee, N.; Lertwattanasakul, N.; Hirata, K.; Suprayogi; Limtong, S.; Kosaka, T.; Yamada, M. Growth and ethanol fermentation ability on hexose and pentose sugars and glucose effect under various conditions in thermotolerant yeast Kluyveromyces marxianus. Appl. Microbiol. Biotechnol. 2011, 90, 1573-1586). K. marxianus also grows on sucrose, raffinose, and inulin at 45° C. under a static condition even when glucose is present unlike S. cerevisiae. Given the benefits offered by K. marxianus, further evaluation of various strains is warranted.